The invention relates to a holding device comprises a cylindrically or prismatically formed porous ceramic blank and means for clamping the holding device in a machine tool for the material-removing machining of the blank to produce a ceramic workpiece. The invention also relates to a method for the material-removing machining of a blank in a holding device and a use for a holding device.
In the production of a ceramic workpiece the enlarged form is worked out of the still porous, premanufactured ceramic blank, which is green or sintered on. The enlarged workpiece is then imperviously sintered. In the process it shrinks to the final form, in which the impervious ceramic workpiece agrees with the mass of a model pattern or a drawing. According to another variation, the ceramic workpieces are worked out of the porous ceramic blank in the original size and the porosity is then closed by infiltration.
Porous ceramic blanks are easy to machine in a material-removing manner as they have not yet attained their final hardness. Therefore, they are also very sensitive in the porous state to all mechanical stresses, such as for example pulling, pressure, bending, torsion, impact and shock stress. However, during machining blanks have to tolerate precisely such stresses. Therefore, these porous ceramic blanks need holding devices especially equipped for their machining which during machining take particular account of the sensitivity of the blanks. The mechanical stresses during machining have to be minimised and reliably diverted into the holding device.
These holding devices are in turn clamped on the rotary shaft of a suitable machine tool for the material-removing machining of the blank for production of the ceramic workpiece. The ceramic workpiece is worked out of the blank with special machining tools. This takes place with at least one respective milling, drilling or grinding tool, for example with a respective milling cutter for the coarse and fine machining. Machining generally takes place in a program-controlled manner but may also take place by copy machining in the original or also in the enlarged state with the aid of a model pattern.
Reference is made to PCT/CH00/00623 for an automatic machine tool for producing basic frameworks for tooth replacement, in particular for tooth crowns and/or tooth bridges of precise three-dimensional form. The basic frameworks, as these worked ceramic workpieces are called in this document, are produced in a machine tool comprising a machine frame or housing, a workpiece carrier with a rotary shaft, at least one digitalising unit, at least one machining unit with the milling cutters and an electronic computing and control unit for all the drive members. A holding device, not shown in detail, for the blank is formed which is fastened to the rotary shaft of a displacement unit with three axes of translation in the x, y, and z direction. Digitalisation of a preparation model clamped on the same rotary shaft and the machining of the blank are carried out on the same machine tool at different times. Prior to machining, the blank the machining paths for the blank are calculated from the determined and stored digitalisation data and a preselected, material-specific scaling factor.
DE A1 4436231 describes a tablet-like blank for a tooth crown, which is embedded in a ring and held by way of this in a clamping device for machining. The purpose of this device is so that only a very small residual quantity of the very expensive materials used in dental technology occurs when the ceramic workpiece is worked out from the blank, in that the diameter of the blank can be reduced. The blanks of exclusively tablet-like shape can no longer be machined with complete design freedom owing to the ring, so even the minimum material consumption is finally also impaired. This holding device with a ring is also intended for the production of tooth crowns and is not suitable for the production of long tooth bridges.
EP A2 0982009 also describes a holder for a blank for producing a tooth replacement part. The blank does not project beyond the support body in any direction, so the freely accessible surface is small. The holder is to protect the blank, in particular during automatic conveying in magazines and the like, which is obviously to the detriment of processing freedom.
The object of the present invention is to provide a holding device of the type mentioned at the outset which, independently of the geometry, provides adequate protection against any excessive mechanical stresses and deformations even for elongate blanks. As large as possible a proportion of the surface of the blank should also be freely accessible for the material-removing machining and the worked out ceramic workpiece should be held securely.